As a high-speed mobile communication standard at an intermediate stage between the third generation (3G) and the fourth generation (4G) in the mobile communication system, the development of Long Term Evolution (hereafter, abbreviated as “LTE”) is pursued. Although the LTE is positioned as an evolution system of IMT-2000 which is a standard of the third generation, the radio interface and RAN (Radio Access Network) architecture of the LTE are drastically reviewed from a view point of smooth shift to 4G, and thus they are largely different from those of the 3G system. In the LTE radio network, with the reduction of delay time and the improvement of communication rate, a data transmission rate of a maximum rate of 300 Mbit/s for downlink and a maximum rate of 50 Mbit/s for uplink is realized.
In contrast to the 3 G communication system where during communication, plural communication links are established for one mobile terminal at one time, the number of link established in the LTE communication system is one. That is, in the LTE communication system, when a mobile terminal moves across cells and a handover takes place, the mobile terminal needs to terminate a connection once with a cell of origin for moving and establish a connection with a cell of destination for moving.
FIG. 1 through FIG. 3 are block diagrams to explain a handover in the LTE communication system.
A LTE communication system 90 illustrated in FIG. 1 through FIG. 3 includes a mobile terminal 95, a communication server 91 to distribute data to the mobile terminal 95, base stations 93a and 93b called as evolved NodeB (eNB) to transmit data by radio from the communication server 91 to the mobile terminal 95, and a gateway device 92 called as Evolved Packet Core (EPC) to distribute the data transmitted from the communication server 91 into either of the base stations 93a and 93b. Additionally, the communication server 91 may be another terminal that is a communication counterpart of the mobile terminal 95. Each of the base stations 93a and 93b communicates with the mobile terminal 95 located in three cells “A” (94a), “B” (94b) and “C” (94c) for which the respective base stations 93a, 93b are responsible. In a state illustrated in FIG. 1, the mobile terminal 95 located in the cell 94b communicates with the base station 93a, and data is transmitted to the mobile terminal 95 via the gateway device 92 and the base station 93a. When the mobile terminal 95 moves from the cell “B” 94b to another cell “C” 94c and a handover takes place, as illustrated in FIG. 2, the base station 93a terminates the connection with the mobile terminal 95, and then as illustrated in FIG. 3, the base station 93b establishes a connection with the mobile terminal 95. In this way, in the LTE communication system 90, an instantaneous interruption occurs between the disconnection and the establishment of connection.
To avoid data loss by the instantaneous interruption, in a radio base station device, it is desired to take measures such as buffering data in a buffer until when a handover is completed in a cell of destination for moving and transmitting the data buffered in the radio base station to the mobile terminal after completing connection of user in the cell of destination for moving. For example, as a technology of temporarily buffering data to be distributed to a mobile terminal, Japanese Laid-open Patent Publication No. 2006-187019 and Japanese National Publication of International Patent Application No. 2002-518958 present communication systems that buffer data transmitted from RNC (radio network controller) in NodeB (radio base station) in the third generation communication network.
A storage capacity required to buffer the data transmitted increases with an increase of a transmission rate of data. Since a downlink communication rate reaches as fast as 300 Mbps in the LTE, it is desired to provide a buffer memory of large-capacity. For instance, if a handover control takes 0.5 second, a buffer memory of approximately 19 MByte is desired per one user, and accommodating 100 users produces a necessity of implementing as much as 2 GByte of buffer memory for the handover. In this way, every time the number of users accommodated in a radio base station increases, a buffer memory for controlling the handover is required, thus increasing a system cost. If the amount of buffering is limited to avoid an increase of cost, a loss of downlink data occurs and may sacrifice the quality of communications.
Here, in the system of Japanese Laid-open Patent Publication No. 2006-187019, the amount of data to be transmitted from the RNC is controlled, thereby restricting the amount of data in NodeB (radio base station). For example, the NodeB or an UE (mobile terminal) monitors the radio quality, and if the radio quality is deteriorated, reduces the amount of data to be caused to flow to the NodeB from the RNC, thereby reducing the amount of data to be buffered in the NodeB. Although the data flow between the RNC and the NodeB is optimized in the system of Japanese Laid-open Patent Publication No. 2006-187019 and the amount of buffering in the NodeB may be reduced, buffering in a higher-level node is desired.
Also, in the system of Japanese National Publication of International Patent Application No. 2002-518958, the amount of data stored in a buffer memory is measured or predicted, and if there is a possibility of an occurrence of buffer overflow, “back pressure” (restriction of transmission) is performed to the higher-level node, thereby restricting the buffer overflow. With this, although the amount of communications in a router may be reduced, similarly to the system of Japanese Laid-open Patent Publication No. 2006-187019, buffering in a higher-level node or another node is desired.